1. Field of the Invention
This invention relates to an optical pickup such as that used in an optical disk drive and, more particularly, to a driving structure for a shaft-slidable objective lens the holder of which is slidable and rotatable and supported by a journal.
2. Description of the Prior Art
Generally, a prior art optical pickup comprises, as shown in FIGS. 7 and 8, an objective lens 1 focusing a light beam, a holder 2 holding an objective lens, a journal 3 for holding the holder 2 slidable axial direction or in the focusing direction 21 and rotatable around the axis tracking direction 22, and an outer core 5 which is a fixed member and on which the journal 3 is mounted. The journal 3 is secured at the center of the outer core 5 by press-fitting or bonding.
The objective lens 1 is held by the holder 2 at a position offset from the journal 3. The holder 2 has a balance weight 11 mounted fixedly at a position symmetrical to the objective lens 1 about the journal 3. In addition, pairs of driving coils for focusing 12 and driving coils for tracking 13 are secured on the outer periphery of the holder 2 at respectively symmetrical positions around the journal 3.
Pairs of focusing magnets 14 and tracking magnets 15 are secured on the outer core 5 at respectively symmetrical positions around the journal 3 so as to be aligned with driving coils for focusing 12 and the driving coils for tracking 13, respectively. The poles N and S of focusing magnet 14 are polarized and magnetized in the axial or focusing direction 21, while those of the tracking magnet 15 are polarized and magnetized in the tracking direction 22.
An inner core 16 is secured to outer core 5, being separated by a gap, from and substantially radially aligned with magnets 14 and 15. The gap accepts driving coil for focusing 12 and driving coil for tracking 13 between inner core 16 and the magnets. The Inner core 16, coil 12, magnet 14, and outer core 5 form a magnetic circuit in the focusing direction 21. Furthermore, inner core 16, coil 13, magnet 15, and outer core 5 form a magnetic circuit in tracking direction 22. A magnetic piece 17 is secured on outer periphery of the holder 2 opposite to the centers of the magnetic poles of focusing magnet 14 in the focusing direction 21 and the tracking direction 22.
The operation of the prior art optical pickup with such an arrangement will now be described. FIGS. 9 and 10 show the relationship between the flux from the focusing magnet 14 and the magnetic piece 17 in a plan view and a side cross sectional view, respectively. Because the focusing magnet 14 is polarized and magnetized in the focusing direction 21, in the curved plane containing the tracking direction 22 as shown in FIG. 9, a flux density of focusing magnet 14 is at a maximum at the central area in the circumferential direction in the gap, and decreases toward the ends of the gap. Because the magnetic piece 17 is placed in this gap, it is subject to a magnetic attraction from the focusing magnet 14, as well as a restoring force to stably hold the magnetic piece at the maximum point of the flux. This restoring force holds the holder 2 at neutral point in the circumferential direction, whereby the objective lens 1 is held at the neutral point in the tracking direction 22.
Furthermore, as shown in FIG. 10, in the cross section of the focusing magnet 14 in the focusing direction 21, the magnet 14 is polarized and magnetized in the focusing direction 21 so that the magnetic piece 17 acts as a part of the magnetic path, and is magnetically attracted to the polarized and magnetized central portion. This attraction acts on the magnetic piece 17 as a restoring force to hold a holder 2 at the neutral point in the axial direction, whereby the objective lens 1 is held at the neutral point in the focusing direction. Such prior art is disclosed in U.S. Pat. No. 4,998,802.
However, the above-mentioned conventional optical pickup must have a structure for polarizing and magnetizing magnet 14 to magnetically float holder 2, on which magnetic piece 17 is secured, as a means for holding objective lens 1 at the neutral point in the focusing and tracking directions. This leads to an increase in cost, and has problems such that a boundary of polarization is varied depending on magnetization accuracy, and that the boundary becomes wide, making it difficult to hold it at the desired neutral position.
In addition, because such a slide-type objective lens driving structure requires a clearance of about 10-20 .mu.m between the bearing hole 4 provided in the holder 2 and the journal 3, it has a problem such that, if any vibration is imparted on the optical pickup, the holder 2 is jerked, which causes severe displacement of the objective lens 1, which the displacement tracking servo fails to follow, so that the vibration resistance is deteriorated. Conventionally, to eliminate this problem, for example, Japanese Utility Model Publication No. Hei 4-2420, published Jan. 28, 1992, proposes a method of pressing the holder against the journal by applying a force on the holder with a resilient support member. Alternatively, the journal is pressed in such a manner that a magnetic plate is overlaid on the driving coil for focusing or the driving coil for tracking, and attracted by the tracking magnet or the focusing magnet.
However, in the former, because the resilient member is a molding of rubber or resin, there arise such problems that it has poor temperature characteristics and ages quickly, and that it has a narrow range of linearity characteristics. In the latter, although such problems do not occur because the magnetic plate is placed in a gap between the driving coil for tracking and the tracking magnet or in a gap between the driving coil for focusing and the focusing magnet, such problems arise that the gap between the coil and the magnet is increased, which weakens the electromagnetic force and the sensitivity in the tracking or focusing direction is reduced, and that the objective lens driving structure must be of a larger size because of the increase of the gap, which causes a problem in reducing the size of the entire device.